1. Field of the Invention
This invention relates to an arrangement for detecting ionization in the combustion chamber of a diesel motor, as well as a measurement device and a calibration device for such an arrangement.
2. Description of the Related Art
There are several known solutions for detecting ionization in diesel motors. In Swedish Patent SE,A,9400603 is shown a solution in which a tube-shaped probe is arranged concentrically around the injection nozzle. Using this tube-shaped probe, it is not possible to access the outer region in the injected fuel plume where the combustion starts and the ionization first occurs. The ideal fuel-air relationship for initiating combustion is found in the border zone between the injected fuel plume and the air mass in the combustion chamber.
In U.S. Pat. No. 4,377,140 as well as published European Patent Application EP,B,190,206 are shown variations in which a probe is included for detecting ionization in the pre-chamber of a diesel motor. Even if the combustion takes place in the pre-chamber, the ionization signal gives worse information about the combustion in the main combustion chamber. In motors with a pre-chamber, a lower efficiency is achieved, and in certain types of motors with a pre-chamber, dual injectors are implemented, one for the main combustion chamber, and one for the pre-chamber.
European Patent EP,B,71,557 shows a variation in which an ion-stream probe sits recessed in the wall of the combustion chamber, and in which a cavity is created in the wall behind a narrow measurement slit around the ion-stream probe. In this case, the fast rate of flow of the combustion gases through the measurement slit into the cavity helps keep the measurement slit free from carbon deposits. This leads to an unfavorable combustion chamber, not only because of the increase of heat loss through the walls of the combustion chamber due to the cavity, but also because a disadvantageous combustion zone is formed in the cavity, which increases the exhaust of incompletely combusted particles. In an alternative embodiment in EP,B,71,557 there is a simple measurement probe in the form of a screw whose end portion lies exposed to the combustion chamber at the level of the roof of the cylinder.
U.S. Pat. No. 5,347,856 shows a solution for use in Otto motors, where measurement of the ionization in the combustion chamber takes place in the spark plug gap. In this text, it is asserted that the size of the central electrode of the spark plug, which is subjected to an electrical voltage, has significance for the amplitude of the measurement signal obtained. It is stated that, by increasing the size of the central electrode, the amplitude of the output signal increases, provided that the applied measurement voltage is kept constant. The explanation for this is that the measurement probe, over which a voltage is applied, is exposed more to the combustion gases and is thus surrounded by more ionized particles, which then increase the signal strength. In this prior art device, a measurement voltage is applied to the central electrode, and the measurement stream that is formed runs essentially between the central electrode and the side electrode, in the periphery of the combustion chamber.
The object of this invention is to correctly detect the beginning of combustion in diesel motors that have direct injection into the main combustion chamber, with minimal lag in the detection. The motor can then be better controlled with respect to the most important parameter, namely, the beginning of combustion.
Another object of the invention is to be able to correctly detect the beginning of combustion even in the cases of so-called pilot-injection, in which only small amounts of fuel are injected into the combustion chamber. Pilot-injection is used both to limit the powerful sound of combustion that is caused by the strong rise in pressure in the combustion chamber and to reduce the combustion temperature and thus even reduce he amount NOx that is formed. The amounts that occur in pilot-injection are on the order of a few percent up to around 20 percent of the total required fuel quantity; there are therefore high demands on the accuracy of the detection.
Yet another object of the invention is to allow wide tolerances in the amount of injected fuel from the diesel injectors, whereby a larger separation between the individual injectors can be accepted. With a more accurate control of the injected fuel amount during pilot-injection, each injector can be controlled individually using feedback information from the combustion. Simpler and less expensive injectors can then be used.
The invention provides an arrangement for detecting ionization in an open-chamber combustion chamber of a combustion motor in which the fuel self-ignites by means of compression. A recess is provided in an upper surface of a piston that moves in the cylinder. Fuel for self-ignition and combustion is injected into the recess. A measurement device has a measurement tip that detects ionization in the combustion chamber. The measurement tip extends into the combustion chamber through a roof of the cylinder and down into the recess of the piston and terminates substantially centrally within the combustion chamber.
In the preferred embodiment, the distance between an innermost end of the measurement tip and the cylinder roof, when the piston is in an upper dead-point position within the cylinder, lies in the interval 20-60% of the distance between the cylinder roof and the upper surface of the piston, and preferably exceeds 40% of this distance.
The recess is substantially annular and rotationally symmetric about a central axis of the piston, with a centrally located protrusion extending upward from an upper surface of the piston. The measurement tip extends into the combustion chamber and down into the recess of the piston so that the distance between an innermost portion of the tip and the centrally located protrusion, when the piston is in an upper dead-point position, is less than the distance between the innermost portion of the measurement tip and remaining surfaces on the upper surface of the piston.
The arrangement preferably also includes an injector that injects fuel directly into and essentially centrally in the combustion chamber and down into the annular recessed piston. The injector has a plurality of outlets directing fuel plumes substantially radially outward from the injector and substantially perpendicular to the cylinder walls. The measurement tip is thereby preferably arranged in the combustion chamber tangentially to the fuel plumes.
The measurement device is preferably substantially rod-shaped and comprises: a) an inner end portion exposed within the combustion chamber, and an outer end portion forming a connection to external measurement equipment; b) an electrically conductive, lengthwise extending member formed as a single piece and a central portion with a diameter greater than diameters of both the inner and outer end portions of the measurement device; c) a longitudinally extending, cylindrical housing having an inner end and a lengthwise extending bore in which the electrically conductive member is located, and a radially inward extending shoulder on the inner end of the housing; d) a cap piece arranged in an outer end of the housing; e) a first insulating bushing arranged in the inner end of the housing between the radially inward extending shoulder on the inner end of the housing and the of the electrically conductive member; f) a second insulating bushing arranged around the outer end portion of the housing between the cap piece and the central portion of the electrically conductive member; and g) an insulating slit located between an inner surface of the housing and the electrically conductive, lengthwise extending member, and which may be filled with an electrically insulating material having high thermal conductivity. The first insulating bushing is preferably arranged concentrically around the electrically conductive member and is provided with a collar that extends in the longitudinal direction towards the measurement tip farther than an innermost portion of the housing.
The invention also provides a calibration device that may be used in conjunction with a compression-ignition motor of the type described above. The calibration device includes: a) a longitudinally extending housing having an inner end portion and an outer end portion; b) an electrically conductive member extending lengthwise within the housing; c) a sheath extending around the electrically conductive member and insulating it from the housing; and d) a pressure sensor having a connecting channel in the housing, a first end of the connecting channel opening into a combustion chamber of the motor and a second end of the connecting channel exposing a sensor element of the pressure sensor to combustion gases in the combustion chamber via the connecting channel.
The pressure sensor is integrated into the calibration device for detecting ionization. The calibration device can be used to calibrate the ion-stream information relative to pressure data for diesel motors. In these diesel motors, detection of ionization is used with an arrangement according to the invention, as well as a measurement device for feedback control of, among other things, the time of injection, the injected fuel quantity, and the balance between the cylinders.
Using the arrangement according to the invention and the associated measurement and calibration devices, an implementation for ionization detection in the combustion chamber of diesel motors is made possible with a very high signal-to-noise ratio. Ionization is detected in direct connection with the combustion zone, so that the starting time of the combustion can thus be detected more exactly; moreover, even small injected quantities of fuel, which result in weak combustion of short duration, can be detected with high accuracy.